text:
At non-controlled fields, what are the regulations concerning "line up and wait"?
Motivation
I am working on best practices for a plane with a very slow constant speed prop. In my experience, it operationally takes 20 seconds or more to go from idle to full throttle (pushing the throttle faster than that causes the engine to overspeed as the prop can't catch up). This is a substantial time to be sitting on the departure end of the runway, and contributes to traffic congestion on busy days.
One time saver is to taxi onto the runway immediately after landing traffic passes overhead. The engine can be spooled up while the preceding plane is landing and taxiing off.
Likewise for departing aircraft as soon as they've begun their takeoff roll.
Question
For standard Part 91 operations at non-towered airports, what FARs are applicable to taxiing onto the runway while other planes are still using it for takeoff, landing flare, or taxiing clear? I know ATC can ask a pilot to "line up and wait", but I'm unaware if there are different FARs in effect when ATC is involved.

I have never heard of a prop that takes 20 seconds to reach full RPM. What aircraft, and what engine/prop do you have?

@MikeSowsun it's not the A/C or engine, it's the MT-7-A electric constant speed prop. It takes 90 seconds to fully feather, and, yes, I have timed that. It does not take 20 seconds to reach full RPM, it takes 20 seconds to pitch sufficiently so as not to exceed redline. I've fixed the question to make that point clearer.

There are no regulations explicitly against lining up and waiting at untowered airports. However, you are turning your back to final approach for the runway you are on for the indeterminate amount of time it takes for the airplane before you to clear the runway, and if there is a problem it is possible 91.13(b) would be applied.

Regarding your issue of long spool-up times, it sounds like your low pitch stop needs adjusting. Refer to "Adjusting the static RPM" in the propeller manual. The low pitch stop should already be in the takeoff position, and it should not be possible to overspeed on the ground.
From the manual:

Check max. static RPM with full throttle and blades in take off
position on ground. The static RPM must be 50 - 100 RPM
below max. permissible RPM (see flight manual).

If your static RPM is over the maximum permissible RPM, then your propeller is adjusted incorrectly according to the manual and your aircraft is not airworthy.

I wish I had an extra credit upvote available for the maintenance troubleshooting suggestion!

@MichaelHall, I as well!

Although with further consideration, I'm not so sure the prop is set up incorrectly. I suspect because this is a motorglider and an in-air wind-milling starts can be required, it might make sense to have a very, very flat pitch available.

@KennSebesta I would think the propeller should have no trouble windmilling in a normal takeoff pitch, and a finer pitch risks the same overspeed condition in flight. But anyway it's something to talk to a mechanic about.

I realized I never followed up on the maintenance aspect of @Chris's answer. Indeed, when talking with MT Props and the airplane manufacturer, both confirmed that the prop should not be able to exceed redline while in static RPM. This has since been resolved.

@KennSebesta Glad to hear it worked out for you! :)


text:
Is it safe to fly a glider when I can't move the stick fully left and right?
This past Saturday, I started training in a Schweizer 2-33A glider.
When I got in the glider for the first time, I found that I couldn't move the control stick fully left and right, because my legs were in the way. I could only move it about halfway. I could move it fully forwards and backwards, though.
Is it safe to fly a glider like this? I asked my instructor about this, and he said it was fine.
Normally, you're not going to use full aileron deflection anyway, unless the glider is on the ground and moving slowly.
On the other hand, if you get into an unusual attitude somehow, then the limited aileron travel could increase the amount of time it takes to recover. But I don't know if that's a realistic concern.

Seems like a question better asked of your instructor.  He might tell you "full deflection is where the stick hits your thigh" -- or he might tell you there's a problem with the glider.

Yes, it is safe.
Well, there is a reason why the stick travels as far as it does. One is the restriction in long-term maximum stick forces of 15 N (see §22.143(b) in the linked PDF), which is easier to fulfil if the stick has more travel. The other is the required roll speed to switch from +45° roll angle to -45° roll angle within 5 seconds for a glider with 15m wing span (see §22.147), which is easier to fulfil if the ailerons have a wider deflection range.
I found myself in the same position on several occasions (after all, I am 6' 6" tall when standing upright - most gliders became only flyable for me after I had fitted a new instrument panel, the 2-33 being a notable exception) and for me it was enough to be able to yank the stick fully to one side when I had lifted my leg off the pedal on that side, so the stick would fit below my knee.
If you want to fly a coordinated turn, the knee on the side where the stick should go will be stretched, in order to push the rudder to full deflection, so then the stick should be able to travel where the knee had been with a centred rudder. This will be the normal case - isolated full aileron without rudder should be the exception.
If you are concerned about unusual attitudes: You will not fly aerobatics, will you? And in order to end a spin, rudder is far more important than aileron, which should be kept centred in a spin.

Agree!  6' 3" myself and I have the same problem.

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text:
Has any accident been caused by poor time management, despite a perfectly done checklist?
This is a question I've heard from an aviation psychologist. I've searched on that but couldn't find a satisfying example. The full question is:
Has there been any accident which was caused by poor time management by the crew, despite a perfectly done checklist?
Thanks in advance.

What do you mean by "poor time management"?

A perfectly done checklist implies good time management.

No you can do your checklists perfectly and still get messed up rushing an arrival.  Time management here just refers to staying ahead of the airplane.

Time management generally means selecting the most important things to do given limited time. And in context of aviation that means following the common maxim “aviate, navigate, communicate”. Which is occasionally called a checklist, but usually not.
Aviate means keeping the plane under control and inside the flight envelope, navigate means knowing where you are and where you are flying, and communicate if there is someone who should know, meaning mainly the air traffic control. And most checklists, except some memory items, only come after these three.
So an accident due to poor time management means the crew got preoccupied with something and basically forgot to monitor the flight path—there is plenty of those—and despite correctly executed checklists means the distraction involved a checklist, probably a non-normal one (dealing with some failure)—I am not sure how close the accidents I can remember of the top of my head come.

Eastern Air Lines flight 401 on 29th December 1972 is probably the best known accident that comes to mind. The crew executed their before landing checklist, but that revealed one issue. And the crew became so preoccupied in troubleshooting it that they failed to notice they inadvertently turned off the altitude hold.
In some sense the Dana Air flight 992 on 3rd June 2012. The crew correctly configured the aircraft for best glide in response to a dual engine failure, but then the pilots kept trying to restart the engines and neglected to prepare for the possibility of forced landing. The checklist for engine failure does mention this though, so ultimately they didn't execute it correctly.
The Distraction in cockpit category on the Aviation Safety Network seems most relevant, but not sure which of the accidents qualify. Maybe this Avianca 727 on 17th March 1988 or this Sky Executive L410 on 21st May 2002?

There is probably more, but it is relatively hard to search.

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text:
Which shapes are classically good for fairing instersections?
I'm fairing a strut/wing interface (pictured in blue in the below screenshots), and I realized while it makes sense to continue in the lift strut's profile to the wing on the shallow angle, I don't have an instinct of how should be the area in the acute angle formed by the lift strut and the wing.
Are there classically ideal shapes for acute intersections beyond some kind of fillet?
A fillet strikes me as something which would be very bizarre here, as the fillet would have to extend both forward and behind the lift strut, adding significant wetted area to the fairing (although at the same time removing some wetted area from the interface).

Normally, you let the fillet radius grow in flow direction. At the nose, the radius is near zero and becomes gradually larger towards the trailing edge.

@PeterKämpf do you have a link to some examples of that? Extending the loft in the direction of the lift strut is easy, but the part that bends back almost 150 degrees isn't making sense to me with the description in your comment.

Chapter 8.4 of Hörner might be helpful.

When in doubt, copy something in use.

Agreed! But only insomuch as there's data showing it works. Is there any for those kind of fairings? That fairing strikes me as aftermarket, and so it could be as much driven by aesthetics as aerodynamics.

Data may be scarce for an ideal fairing, because when low drag really matters, the airplane doesn't have struts at all.  Can we find an upper bound for the drag of a poor fairing, as a fraction of the whole strut's drag?


text:
When reporting roll attitude, is that relative to the horizon or relative to the Earth frame?
I'm trying to figure out if what I've always thought of as roll from the cockpit is not in fact roll at all.
The Tait-Bryan convention defines RPY (roll, pitch, and yaw) as the Euler angles from the Earth-fixed coordinate system. However, when I'm in the cockpit, looking at a gyroscope, I'm seeing my wings relative to the horizon. This is what I have imagined the attitude indicator indicates.
At 0 degrees of pitch, Euler and gyroscope correspond perfectly, but they diverge as pitch gets farther from 0. It's not a huge difference, though; for instance an Euler pitch angle of 40 degrees and an Euler roll angle of 60 degrees leads to a 53 degree angle made between the wings and the horizon.
(For the mathematics of why, this has to do with the difference between rotations about a body-frame axis and rotations about the world-frame. While the gyroscope is reporting the difference between the horizon and the wings, the Euler angles are defined about the world-frame. The above discrepancy can be confirmed by looking at the horizontal basis vectors of the {1,2,3} sequence where theta = 40 and phi = 60. When examining the body coordinate frame's y-axis the divergence is clear.)
So when I talk about roll as a pilot, which am I actually talking about? I'm assuming roll relative to the horizon, but I'm really not sure anymore. I realize it doesn't particularly matter, since if you're 40 degrees nose down and at 60 degrees of bank, the 7 degree difference isn't really your critical problem at that moment... But I am curious as to which I should use when reporting RPY values from an autopilot.

Excellent question, perhaps a dup of What are the exact meanings of roll, pitch and yaw? (you decide). See also bank vs roll angles.

@mins thatnks for those links. I think quiet flyer's discussion of attitude, and its various slippery definitions, helps. I don't think it quite clears up the confusion yet. (Part of that is because I don't see much consensus around his claims, although I think they do make a lot of sense.) I feel like there must be a reference source which specifically addresses the two, and gives mathematical conversions between the pilot perspective and the inertial frame perspective. Maybe the pilot is just technically wrong in calling it "roll", but the error is tiny so .

Not visalizing the question yet; at (say) 40 degrees of pitch, 0 roll is the same in either frame, but 60 degrees of roll differs; what about 90 degrees of roll - is that case different between the two frames of reference?

Correct. And at 90 degrees of roll, the discrepancy returns to 0. There's a maximum somewhere near, but exactly at, where Euler angle roll and pitch are the same.

When you say "reporting" and "talking about it" as a pilot, what do you actually mean?  Because while I broadly understand the nuances, it has never been an issue worth mentioning, even in briefing and debriefing dynamic aerobatic and combat maneuvers, much less an autopilot.  Also, consider 90 degrees nose up or down - you can still roll the airplane but what is the frame of reference then?

@MichaelHall I don't understand your question. It might never have been an issue worth mentioning, but it's still a divergence in the notation of the flight dynamics vs the cockpit view. I wish to understand this better.

I'm just trying to clarify whether by "reporting" you mean communicating detailed autopilot numerical software data, and "talking about it" means discussing with engineers how to make the autopilot work properly WRT this data.  I assure you I am not trying to be pedantic or petty, just trying to put this query in context.  Because at small pitch angles aren't the frames of reference close enough to not matter?

@MichaelHall, by reporting, I mean if I'm in the cockpit, I look at my attitude indicator, and say, e.g., "I'm at 10 degrees of pitch and 20 degrees of roll." In that case, I am reporting the roll attitude as the angle that the airplane silhouette's wings make with the horizon, and the pitch as the bar the silhouette's nose is touching.

Hmmm, I'm guessing on most relevant flight instruments (or HUDs), if you're 60 deg or more nose-down, the artificial horizon is not within view anyway.

Ok, I won't belabor this any more, but at the risk of sounding flippant, do you actually voice the words "I'm at 10 degrees of pitch and 20 degrees of roll"? If so, to whom, and for what reason?! My point is that when you "talk about roll as a pilot" the sort of distinctions you are asking about are largely irrelevant unless you are involved in flight test or engineering. It's an intelligent question, (I think...) but the way you've framed the issue is puzzling because there's no clear problem statement or practical reason for ordinary pilots to really care about or use such terminology.

However, when I'm in the cockpit, looking at a gyroscope, I'm seeing my wings relative to the horizon. This is what I have imagined the attitude indicator indicates.

It is correct. The Earth-Fixed frame is partially constructed by the AI: The AI gyroscope maintains its rotation in a local horizontal plane, about its vertical axis, and therefore can sense a change in both in pitch and roll independently as seen on this diagram:

Source.
When the aircraft pitch angle changes, the gyro rotates the orange gimbal about the horizontal pitch axis (actually the gimbal is fixed, the rest rotates). The pink roll gimbal doesn't move, but the pink pin (which should be orange, I guess) moves up or down when rotating with the orange gimbal using the aperture in the gimbal. This moves the arm and the dial up or down (but doesn't change the roll angle).
When the aircraft roll angle changes, the gyro rotates the pink roll gimbal, and the dial with it.

an Euler pitch angle of 40 degrees and an Euler roll angle of 60 degrees leads to a 53 degree angle made between the wings and the horizon.

Let use this body-fixed frame, a right-handed one common in aerospace:

Source: Introduction to Aeronautics
The standard chain of rotation is an intrinsic z-y-x on the body frame (the 3-2-1 Tait-Bryan convention) for yaw, pitch and roll in this order. In this convention the final orientation of the body, is obtained this way:

The body frame is initially aligned with the Earth frame.

The body frame is rotated about its z-axis (vertical axis) by the yaw angle ($\Psi$). This changes the orientation of the other axes of the body frame, let's call the new orientations x' and y'.
The corresponding transformation matrix is:
$$\begin{bmatrix}
  \cos \Psi & \sin \Psi & 0 \\
  -\sin \Psi & \cos \Psi & 0 \\
  0 & 0 & 1
  \end{bmatrix}$$

The body frame is rotated about y'-axis by the pitch angle ($\theta$). This changes the orientation of the other axes, let's call the new axes x" and z". The rotation matrix:
$$\begin{bmatrix}
  \cos \theta & 0 & -\sin \theta \\
  0 & 1 & 0 \\
  \sin \theta & 0 & \cos \theta
  \end{bmatrix}$$

The body frame is rotated about the x"-axis by the roll angle ($\Phi$). The rotation matrix:
$$\begin{bmatrix}
  1 & 0 & 0 \\
  0 & \cos \Phi & \sin \Phi \\
  0 & -\sin \Phi & \cos \Phi
  \end{bmatrix}$$

In this situation the body frame as been rolled by the roll angle, whatever the previous yaw and pitch rotations. For example if yaw and pitch are 0°, the body frame would still be rolled by the same angle. This shows the roll angle is always the angle between the y-axis (y') of the body frame and its projection on the horizontal plane (xy plane of the Earth frame) so:

When reporting roll attitude, is that relative to the horizon or relative to the Earth frame?

Relative to both, as these two concepts are coincident.
Transformation matrices
Let select the Earth frame to be a flat-Earth NED, and the rotations yaw ($\Psi$), pitch ($\theta$), roll ($\Phi$) associated with a 321 Tait-Bryan order like above. The two transformation matrices are indeed the simple composition of the individual rotations and it inverse:
For NED to body-fixed:
$$
\begin{bmatrix}
c \theta c \Psi &
c \theta s \Psi &
-s \theta \\
s \Phi s \theta c \Psi - c \Phi s \Psi &
s \Phi s \theta s \Psi + c \Phi c \Psi &
s \Phi c \theta \\
c \Phi s \theta c \Psi + s \Phi s \Psi &
c \Phi s \theta s \Psi - s \Phi c \Psi &
c \Phi c \theta
\end{bmatrix}
$$
and for body-fixed to NED (in this case the inverse is the transpose):
$$
\begin{bmatrix}
c \theta c \Psi &
s \Phi s \theta c \Psi - c \Phi s \Psi &
c \Phi s \theta c \Psi + s \Phi s \Psi \\
c \Phi s \Psi &
s \Phi s \theta s \Psi + c \Phi c \Psi &
c \Phi s \theta s \Psi - s \Phi c \Psi \\
-s \theta &
s \Phi c \theta &
c \Phi c \theta
\end{bmatrix}
$$
Were c means cosine and s means sine. I copied the matrices from USAF Equations of motion which also provides a formal explanation in §13.4.1, similar to my answer. It may be easier to follow.
Euler vs Tait-Bryan
Euler formulated a method to describe any 3D solid angle by a sequence of 3 rotations over the 2 axes. So the first and last rotations use the same axis (with two cases for the last rotation, original or rotated axis). Tait-Bryan is another method where the 3 different axes are used once. Tait-Bryan compositions are more simple when describing the aircraft attitude.
See Methods of Representing Solid Angles for more details.

I need some time to go through the rest, but the projection onto the WCS yz-plane of the line made by the 2nd basis vector of the body-fixed coordinates as seen from the WCS is not parallel to a line on the yz-plane tilted by the euler angle roll value. (I know that's a word salad, sorry!) Formulating a rotation matrix with 0 yaw and looking at the second row makes this apparent.

It is, though, important to note that the WCS as seen from the body frame is represented by the column vectors of the rotation matrix. This is perhaps precisely what the AI is indicating, which is how the pilot sees the world, and not how the world sees the pilot.


text:
Why did biplanes and triplanes go away?
Why did multi-wing aircraft (two or three stacked wings) eventually fade off to single-wing aircraft?

How does this affects fuel consumption

I don't think this should be marked as a duplicate. The other question asks why biplanes used to be more prevalent; this one asks why monoplanes are more prevalent now. (As of right now, this question also asks why biplanes used to be more prevalent, but I submitted an edit suggestion to remove that paragraph.)


text:
Is there any airspace where you can fly without a flight plan?
Can pilots go "offroading"? In other words, is there an airspace where you don't need a flight plan? Or maybe just a flight plan to go there and then "joyride?"

Most of the amateur pilots I know offroad, VFR, outside controlled airspace (usually class G).  Some don't even have radios.

I wonder if this question originated from https://twitter.com/tommy_oswalt/status/1387048285693628421?lang=en

Related: https://aviation.stackexchange.com/questions/5148/are-all-flights-planned

Note: I fly in EASA-land (EU) but I'm assuming the situation should be similar in the US and other countries
As @757toga described perfectly, there are situations where a pilot does not need a flight plan, mostly in low altitude, uncongested areas ("uncontrolled airspace"), in good weather at daytime. This varies country to country.
Equally importantly, however, you can go "joyriding" on a flight plan in most circumstances, sometimes even in controlled airspace.
In fact, 90% of the PPL practice flights in my area are on a flight plan, but they list some remark like "MANOUVER PRACTICE NEAR [village] BETWEEN [min-altitude] AND [max-altitude]". There's nothing wrong with that in uncontrolled airspace, it's actually usually much preferred than not filing a flight plan at all, for obvious reasons. *
In many situations, you may even be able to pull this off inside controlled airspace, if the controller permits you. If you'd like to do that, you can either just call them on the phone in advance, or you may need to get a written permit, depending on the place. But if traffic and other factors allow, the controller can allocate a block of airspace for you, inside which you can pretty much "do what you want". This is also how some special operations are done like photography,  airshows, etc.
* Advantages of having a flight plan even when not required include:

Lets controllers and thereby other traffic know about your intentions, which helps you to see and avoid eachother.
In case you get into an accident or simply "disappear" from radar, the flight plan lets the authorities know about your existence so they can notify search and rescue services. They also know your approximate intentions, and important information like # of people and fuel on board.
In my country in addition to the above if you don't have a flight plan and you require search and rescue for any reason, you are financially liable for those services.
If you accidentally get into bad weather and would like an IFR clearance to penetrate clouds you can't get one without a flight plan (at least here under EASA rules). You can technically file a flight plan from the air, but that's one extra step in an already time-critical phase.

This is a good answer (I upvoted it), and can be improved with replacing "for obvious reasons" with explaining the reasons.  What's obvious to one person is often not obvious to another... especially when it comes to this topic.

Good point, updated the post.

FWIW, FAA-land seems to be unusual in that we can fly VFR (nearly) anywhere without a flight plan.  The first time I ever filed one at all was during IFR training.

To be a little less technical, and simply answer the specific question, yes, there is a ton of airspace in the USA where you can fly without filing a flight plan, like almost all of it under 18,000'. I've logged all 800+ hours of my flight time without filing a flight plan.

Someone always knows where I planned on going and when I'm expected, and I always check in with them when I get there. All of my 800+ hours are VFR. If I need to transition various airspace, I request flight following from ATC. At that point, I'm in the system and they know where I am. Now, with ADS-B out, they know even more about who I am and where I am. In my opinion, filing a VFR flight plan doesn't do much more than give them a route to search if you don't show up at your destination.

There are plenty of places in the Western US that are below radar/ADS-B coverage. Every time I fly near the Sierras, I expect ATC to drop me from flight following. I've also been denied flight following when ATC is busy. VFR flight plans make sure that there's still someone who will look for you, even if ATC drops you from the system.


text:
How often is ILS accuracy checked?
I imagine checking ILS accuracy by measuring ILS signals at various points along the approach path is routine maintenance, although I don't know how it is done. How often must this kind of check be done to ensure the ILS is still accurate enough?

Related: https://aviation.stackexchange.com/questions/35341/what-is-flight-inspection-as-provided-by-cobham-aviation-services/

I don't know the exact answer (hence why this is just a comment) but from experience the ILS is monitored continuously, checked by a vehicle driving down the runway weekly and calibrated (by using an aircraft) twice yearly.

I think there is a difference between inspection and monitoring. Your use of the word “checked” may be too ambiguous.

The FAA has a Flight Inspection Manual which describes the checks that must be done and how often. Table 4.1 shows that an ILS must be flight checked every 540 days. Section 15 describes what happens during one of these checks. It's quite an extensive inspection!

Note that many countries (at least in Europe) seem to be working with a 180 day interval for ILS calibration.

@J. Hougaard - Are there any specifics on routine monitoring?

@DeanF. Been digging around a bit, doesn't seem like the ICAO provides that many specific intervals (although they do describe quite extensively what exactly has to be done - see DOC 8071). So it would seem it is largely up to member states to determine how often calibration is needed. As for monitoring alone, that is done continuously

Considering that monitoring and inspecting are different, can one of you define or give the parameters, techniques, or tasks/items/steps to accomplish for monitoring?

@DeanF. The equipment and radio beams are monitored with physical equipment on the airfield. I am not an electrical engineer, don't ask me ho exactly it works

There's some discussion of navaid monitoring in JO 7210: https://tfmlearning.faa.gov/Publications/atpubs/FAC/0305.html

@J.Hougaard Given their announcements for 2019, the German ATC checks the ILS in FRA quite often (several times a year).

@PerlDuck They probably don't do all runways at the same time, hence the large number of announcements

Each ILS gets test flown as well.

@J.Hougaard When possible / needed all runways are done in one go. Individual runways then are done after adjustments to the ILS system or modifications to the direct environment (to assess impact of possible signal reflection interference). Some runways in Frankfurt have dual ILS installations with two different glide slopes (3.00° & 3.20°), so there is quite a lot to calibrate.


text:
Do Federal Regulations really require compliance with all crewmember instructions?
On commercial flights, we're frequently told that all passengers are

"required to comply with lighted signs and placards, and all crew member instructions".

But when it comes to the CFRs, the closest I can find is 14 CFR 121.571

(a) Each certificate holder operating a passenger-carrying airplane shall insure that all passengers are orally briefed by the appropriate crewmember as follows:
    (1) Before each takeoff, on each of the following:
      (i)Smoking. Each passenger shall be briefed [...] This briefing shall include a statement that the Federal Aviation Regulations require passenger compliance with the lighted passenger information signs, posted placards, areas designated for safety purposes as no smoking areas, and crewmember instructions with regard to these items.
      (iii) The use of safety belts, [...] This briefing shall include a statement that the Federal Aviation Regulations require passenger compliance with lighted passenger information signs and crewmember instructions concerning the use of safety belts.

These rules seem to say passengers only have to follow instructions regarding Smoking, and Seatbelts.
Is there some specific, Federal Rule1, other than catch-all 91.3, that requires general compliance with all instructions?
What if I want to leave my tray table down?  Or keep my electronics on? Or put my feet in the aisle?

1Not a contract of carriage with the airline

If you want to avoid being removed from the aircraft, yes, you need to do what they’re telling you to do. The FAA & the airport police can sort out what you’ll be charged with (“interfering with flight crew” is usefully broad), but no Captain I know would hesitate to return to the gate to deplane a passenger who wasn’t following the FA’s instructions.

I think the answers for the old question do not sufficiently answer both, so I would prefer to keep the question open.

You may or may mot be told that on commercial flights, but there is no requirement to include that in the oral breifing.

The Southwest standard safety briefing says "Federal Aviation Regulations require Passenger compliance with the lighted Passenger information signs, posted placards, and Crew member instructions including information on seat belts and smoking" which seems like a nod to these regulations.  https://quizlet.com/36754761/pas-for-swa-flash-cards/

There are also rules under §121.589 Carry-on baggage which includes (e) Each passenger must comply with instructions given by crewmembers regarding compliance with paragraphs (a), (b), (c), (d), and (g) of this section. Airlines also have OpSpecs that they are required to comply with. If you refuse to do or stop doing something that prevents the crew from complying with the OpSpec that could be construed as interference under §121.580 Prohibition on interference with crewmembers.

I found an overview from a lawyer on what "interfering" actually means. The short version is that there's no regulation or law that says "you must obey all instructions". Instead, they say "you must not interfere" and that's interpreted very broadly.
She based the article on one regulation and one law:
14 CFR 121.580 (see also 91.11 and 135.120)

No person may assault, threaten, intimidate, or interfere with a crewmember in the performance of the crewmember's duties aboard an aircraft being operated under this part.

49 USC 46504

An individual on an aircraft in the special aircraft jurisdiction of the United States who, by assaulting or intimidating a flight crew member or flight attendant of the aircraft, interferes with the performance of the duties of the member or attendant or lessens the ability of the member or attendant to perform those duties, or attempts or conspires to do such an act, shall be fined under title 18, imprisoned for not more than 20 years, or both. However, if a dangerous weapon is used in assaulting or intimidating the member or attendant, the individual shall be imprisoned for any term of years or for life.

The lawyer's analysis is that not following an instruction would be a violation of the regulations (civil penalty). If you go further and physically interfere with a crewmember that would be assault (criminal penalty).
When it comes to instructions specifically, she says that "interference" can be interpreted very broadly (emphasis mine):

Almost any offensive or disruptive behavior that distracts the crew
can be considered interference, such as:
[...]

disobeying repeated requests to sit down, return to your seat, or turn off an electronic device

Her basic advice is to follow all instructions if possible and never touch a crewmember:

Any time you disobey a crewmember’s instructions, you run the risk of violating federal law. But civil penalties and criminal prosecutions usually result only when passengers repeatedly ignore, argue with, or disobey flight attendants; or when they act out in a way that is dangerous.

You can see the FAA's enforcement statistics on this, by the way. Interestingly, it seems that the number of cases fell sharply in 2005 and trended down since then until 2020 (with some above-average years), but has since sharply increased, likely due to violations of mask mandates.

According to FAR/CFR
§ 121.577 Stowage of food, beverage, and passenger service equipment during airplane movement on the surface, takeoff, and landing.
(a) No certificate holder may move an airplane on the surface, take off, or land when any food, beverage, or tableware furnished by the certificate holder is located at any passenger seat.
(b) No certificate holder may move an airplane on the surface, take off, or land unless each food and beverage tray and seat back tray table is secured in its stowed position.
(c) No certificate holder may permit an airplane to move on the surface, take off, or land unless each passenger serving cart is secured in its stowed position.
(d) No certificate holder may permit an airplane to move on the surface, take off, or land unless each movie screen that extends into an aisle is stowed.
(e) Each passenger shall comply with instructions given by a crewmember with regard to compliance with this section.

Title 49 U.S. Code Section 46504 states that interference with the duties of flight crew members and attendants can result in a fine and up to 20 years in prison.  This means anything that interferes, including refusal to stow a carry on, non compliance with safety briefing instructions, refusal to be seated when the seat belt sign is on, any disruptive behavior.  Nothing like this is tolerated and Will result in removal from the aircraft by the crew, customer service or armed airport security.

To inject a little bit of realism here - sure, if a flight attendant tells you to stop wearing the color brown, you're absolutely in the right to politely refuse (although you might want to be sure of their reason for telling you that before you do).  However, when a flight attendant gives you an instruction, it's almost always going to be safety-related; for example, stowing the tray tables for takeoff and landing is required to facilitate an evacuation if one is required.  Keeping your seatbelt fastened during taxi not only keeps you safe, but keeps you from turning into a human projectile if the pilot has to stop suddenly (at low speeds, aircraft brakes are able to bring the aircraft to a stop almost instantaneously).  So as a practical matter, you should comply simply because it's in your best interest.  As a legal matter, FAR 91.11 also applies in almost every instance.  The FAA requires an air carrier to follow certain rules (for example, tray tables up for takeoff and landing.)  The flight attendant's duty in this situation is to ensure tray tables are up.  If you're refusing, then you're interfering with their performance of that duty.  I think the problem in interpretation arises from people simply not knowing why a crewmember is asking them to do something.  In typical American fashion, it doesn't matter if a passenger knows nothing about aviation, if the answer isn't completely apparent, then it must be a conspiracy.

Further to this, 91.107(3) requires the passenger wear of seatbelts throughout surface movement, takeoff, and landing.  So the flight attendant is reminding you to do something you are legally required to do.

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text:
Would this "being paid while flying" scenario be considered "flying for compensation"?
The difference between a private and commercial pilot's license hinges on this critical ability to expressly receive pay for flying; a PPL is allowed to fly "in furtherance of a business" but cannot be paid to fly a plane, while a CPL can. That brings up an important question; in today's corporate world, there often isn't a strict "hours for pay" exchange; you get a salary, for which you perform the duties of the job (which do not necessarily mean being there 8 hours a day). So,  what's the difference between being paid while flying in furtherance of a business, and being paid for flying?
A simple example (I hope): I'm a salary man, and my workplace happens to abut a public air park in Dallas. A coworker of mine needs to get to Waco to have a short face-to-face meeting with a client (teleconferencing won't do), and my boss knows I'm a PPL and own a fairly speedy Cherokee 235 (cruising speed 135 mph). The forecast for tomorrow is perfect flying weather, so he proposes this deal; I commute to work in my plane tomorrow instead of driving. After morning coffee and e-mails, my coworker and I will head over to the air park and fly out from there to Waco in my plane and be picked up by the client. We'll have the meeting, be driven back to the airfield, and I'll fly us back to Dallas so he can drive home.
For this, I'll get paid for the workday as I normally would, no vacation or other leave time required to be used. My boss also suggests that I can expense my fuel costs and any landing fees, like I would be able to do with my gas and parking fees if I drove. My boss gets two people face-to-face with the client faster than if we drove, with less time off our normal duties than if we flew on a regional jet (and much cheaper), and I get paid to fly my plane without being explicitly hired to do so.
The questions:

Can I accept the deal at all, or will the fact that I'm technically being paid while flying be considered the same as being paid for flying even though no additional money specifically for my yoke time is changing hands?
What portion of my out-of-pocket costs can legally be expensed?

All my fuel and landing fees incurred between taking off from and landing back at my home field (which I otherwise would not have incurred)?
100% of costs from when I take off at the air park near work to when I return there (the portion of the flight in furtherance of the business)?
50% of "non-commuter" flight costs (gas to and from Waco and the landing fee there, but not the fees at the air park or gas to and from my home field)?
No expenses allowed (because that would be "compensation for flying")?

Would any of this change if I were an hourly employee doing this "on the clock" as opposed to being salaried?
Would any of the above answers change if my boss wanted me to rent a speedier plane (that I was rated for of course)? Would I be able to expense some or all of the rental?
I'll try not to make this an off-topic "legal advice" question: if I refused the deal because I thought it would violate the restrictions of my PPL on flying for pay, and I was summarily fired, should I be talking to a lawyer about a wrongful termination suit?

The short answer is that you'd need a commercial certificate, and potentially a commercial operator's license, to do what you're describing. Standard disclaimer: IANAL, and if you wanted to do anything like this, you really should find an aviation lawyer and get answers from them in writing!
The rules that cover this are contained in FAR 61.113, quoted in relevant part:

§61.113   Private pilot privileges and limitations: Pilot in command.
(a) Except as provided in paragraphs (b) through (h) of this section, no person who holds a private pilot certificate may act as pilot in command of an aircraft that is carrying passengers or property for compensation or hire; nor may that person, for compensation or hire, act as pilot in command of an aircraft.
(b) A private pilot may, for compensation or hire, act as pilot in command of an aircraft in connection with any business or employment if:

(1) The flight is only incidental to that business or employment; and
(2) The aircraft does not carry passengers or property for compensation or hire.

(c) A private pilot may not pay less than the pro rata share of the operating expenses of a flight with passengers, provided the expenses involve only fuel, oil, airport expenditures, or rental fees.

To point 1, the answer is no. You're being compensated for your time, which is spent flying. If you were alone, 61.113(b) would allow you to be compensated - but that only applies if the aircraft does not carry passengers. Since the 61.113(b) exclusion doesn't apply, you're stuck with 61.113(a)'s prohibition of flying for compensation or hire.
You make an interesting point at the beginning about salary covering strange hours. If you really were paid just for your productivity, and you had an arrangement where you could just go take a couple of hours off in the middle of the day to go flying (or any other personal activity) without burning leave or affecting your pay, you could claim that you weren't being compensated for your time. I might write to the local FSDO for clarity on that before actually doing it, though.
If you were an hourly worker (#3) and were not paid for the time you were flying, that would eliminate some of the compensation. Note, however, that the FAA constructs "compensation" very broadly, to include good-will from others. If you made this flight in order to make your boss happy, you're being compensated! The source of the incentive to fly would be important: if you and a co-worker had to get to a destination, and you suggested, "Let's fly there!", you're under much better grounds than if your boss told you to.
The second point would depend on your normal work situation. Let's look at two portions of the flight:

The portion from home to work, and from work back home: In this case, you could be compensated for the flight under 61.113(b) - both in salary and costs, as long as your employer has a policy of reimbursing staff for their commuting costs.
The "non-commuter" costs: You could ask for 50% of the costs of the flight under 61.113(c) (see Mangiamele interpretation) - but from your co-worker, not your employer. Where they get that money is up to them. One important point: 61.113(c) requires "bona fide common purpose" -- you and your co-worker must each have a reason, beyond the flight itself, for going to the destination airport. If you weren't required to meet the client (or do something else that requires you to physically be in Waco at that time), you could not even share the costs of flight. Note that you can't be paid or reimbursed for your 50% — you have to eat that cost yourself.

To #4, the owner of the plane doesn't matter that much here, for a private pilot. The question of who provides the plane becomes very important in common carriage, but that's a different question.
To your last point, I'm not a lawyer - but I imagine that being fired for refusing to take an illegal action would be wrongful termination.
Further Reading:
An FAA Article on cost sharing
Mangiamele Interpretation: a letter from the FAA Office of Legal Counsel discussing this topic

One other thing - I feel like I read somewhere that in the "non-commuter" costs, your co-worker can't be reimbursed for their 50% they give you - that being indirect compensation - but I can't find a citation anywhere.

If you were alone, 61.113(b) would allow you to be compensated - but that only applies if the aircraft does not carry passengers I think 61.113(b) does apply when carrying passengers just not when carrying passengers for compensation or hire. If you carry company personnel on a business trip, you can still be paid your salary (not related to service as a pilot). But it is 61.113(c) that says A private pilot may not pay less than the pro rata share of the operating expenses of a flight with passengers, provided the expenses involve only fuel, oil, airport expenditures, or rental fees.

@fjch1997 No - that's the whole point of 61.113(b)(2). If you're paid for the time spent flying, that counts as compensation. See Hurst (2013).

What you describe in your example at best is a corporate flight department that operates your plane (with the proper legal arrangements) and you fly the plane.  You need a commercial certificate for this -- you won't be able to argue your company salary isn't in some part compensation for the flying.
At worst, you operate the plane and are a part 135 carrier which requires an operating certificate under 14 CFR 119 and compliance with 14 CFR 135.  Of note you'll need a commercial certificate and 500 hours total time to fly the plane in VFR conditions and you'll need an instrument rating for the commercial certificate to be useful.
You can fly yourself to and from meetings, but do not offer to do this for co-workers and do not feed the idea to your boss so that he proposes it.  Whatever you do, join AOPA and consult their lawyers to get a real interpretation of how the regs apply to your possible arrangements and if there is still grey area, consult the FSDO for their opinion.

Where is the 500 hour number coming from? I though the CPL requires only 250 hours.

@Pugz part 135.

What section please? I haven't got to my CPL yet... Almost there.

@Pugz it isn't a CPL requirement. 135 regulates on-demand commercial operations and 121 regulates scheduled commercial operations.  Each have their own specific requirements above a CPL to engage in that kind of flying. §135.243
Lists the requirements to serve as PIC in a part 135 operation.

You CAN use a plane to carry passengers for business purposes as a PPL only when the flight is incidental to the business.  Taking a coworker to a meeting that you both need to attend in order to see a client with which you have (or are seeking to establish) a business relationship is a valid, legal use of an airplane within the confines of PPL Privs & Lims.
As you describe it (your purpose is strictly to fly the plane) this is a charter, plain and simple.
Now, if you ALSO had to meet this client you would be able to demonstrate a legitimate "common purpose" for the flight and you'd be fine.  Use of a plane is no different than giving your buddy a lift in your car, on the handlebars of your bike, or any other mode of transportation PROVIDED your "value-add" is not just as an airplane pilot.
If you have no business other than just acting as the pilot, you are violating the regs.  Read the Haberkorn Opinion and go from there.
Question 2: use the IRS guidelines for reimbursement.  They're reasonable.
I would not expense any of it through the company unless I talked to a tax lawyer first.  I'd just write the portion of hours flown for work off on my personal income tax return (fuel, fees, insurance, hangar, etc)...but that's me.  Again, talk to a lawyer.
Question 3: no.  The mere fact that you're earning flight time crosses the compsensation threshold.  The feds will make that violation stick...they've done it before.  Earning goodwill from the boss may also be compensation (as mentioned in another answer).
Question 4: that just makes it seem more like a charter.  The fewer aspects of the flight that you, as PIC, control the closer you get to charter territory.
Question 5: yes.  You can't be fired for refusing to break laws.

Your interpretation in your first paragraph seems to directly contradict the FAA's interpretation as cited in NathanG's answer: since you are transporting people to the meeting, the allowance for the flight to be conducted for compensation or hire (i.e., reimbursement) under 61.113(b) does not apply -- at least insomuch as mileage payments or such from the company…

…reading between the lines (as best I can, not being a lawyer): It might be kosher to make the flight under the conditions you describe, but any form of
"compensation" would make it a regulatory no-go.

Which is why I would not seek to expense the costs of the flight through my employer and instead write it off as a business expense, which it would be.  I call this out in my answer.

Question 5: Depending on the state, you can be fired for any reason or no reason at all.

True, but depending on the circumstances he may have a decent case for wrongful termination.

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text:
In a flight plan, does it make any difference to write an increase of the flight level instead of writing a cruise climb?
According to ICAO 4444, a change of speed or level should be written in item 15 of the flight plan according to these rules:

Change of speed or level (maximum 21 characters)
  The point at which a change of speed (5% TAS or 0.01 Mach or more) or
  a change of level is planned, expressed exactly as in (2) above,
  followed by an oblique stroke and both the cruising speed and the
  cruising level, expressed exactly as in (a) and (b) above, without a
  space between them, even when only one of these quantities will be
  changed.
Examples:
LN/N0284A045
  MAY/N0305Fl80
  HADDY/N0420F330
  4602N07805W/N0500F350
  46N078W/M082F330
  DUB180040/N0350M0840
Cruise climb (maximum 28 characters)
  The letter C followed by an oblique stroke; THEN the point at
  which cruise climb is planned to start, expressed exactly as in
  (2) above, followed by an oblique stroke; THEN the speed to
  be maintained during cruise climb, expressed exactly as in (a)
  above, followed by the two levels defining the layer to be
  occupied during cruise climb, each level expressed exactly as
  in (b) above, or the level above which cruise climb is planned
  followed by the letters PLUS, without a space between them.
Examples:
C/48N050W/M082F290F350
  C/48N050W/M082F290PLUS
  C/52N050W/M220F580F620

Does it make any difference to write an increase in the flight level instead of writing a cruise climb? Are they different flight maneuvers?

I just can guess based on wording. Isn‘t a cruise climb the initial climb, usually happen soon after take off. While the change in fligh level is a dedicated change of the flight level e.g. flying lower or higher behause of fuel efficieny, mountain, visibility or other reason.

They are different things. A cruise climb allows you to continually climb at your own rate to achieve maximum efficiency. This is what Concorde used to do. However this requires ATC to "give you" all of the levels from the bottom to the top of the climb, which is impractical with the volume of traffic.
Instead, we use set levels. We can "step climb" to another level when we become light enough to do it, and this way ATC only has to block off one level at a time for us.


text:
Should I carry a parachute on GA flights?
The Eurofox aircraft that I have a share in does not have a parachute option fitted and there is no appetite for this at my club.
Would it be safe / reasonable / useful to just wear a parachute when flying? I'm thinking it's use case would really only be mid-airs or structural failure.
Furthermore, how much safer would this make each flight, statistically?
Thanks

Could you operate the parachute? -- Just a though: if you have structural issue, probably you cannot exit properly the aircraft. Air force uses different method to "exit" (so parachutes seems to be effective), so probably it may depend on aircraft type: what do your club use?

Sure, go ahead, why not?  Pilots spend most of their time looking down at screens these days, so mid-air collisions are becoming ever more likely.  PS re "Furthermore, how much safer would this make each flight, statistically"  -  well, let's see.  If a given flight has a 2% chance of fatality (yes I know that's not a realistic number) and wearing a parachute drops it to 1%, has the flight become 50% safer or is the correct number 100* ((99/98)-1) = 1.02% safer?

@GiacomoCatenazzi a Eurofox

Can you actually wear a parachute when sitting in the plane seats, without being ridiculously uncomfortable? Most planes aren't designed for parachute wearers.

Voting to close this as opinion based. There is no way to conclusively answer this question, and the risks mentioned-- midair and structural failure-- are almost vanishingly small compared to all the other reasons pilots crash. (Unless flying a glider, or doing aerobatics, but based on the description of the Eurofox club plane it doesn't sound these edge cases apply.)

Statistically, you're far more likely to have a spin/stall accident turning final than a midair. But then again statistically you're much more likely to crash your car on the way to the airfield than the plane. Statistics are somewhat pointless here.

How often do you think airplanes spontaneously come apart or hit each other in mid-air? These are things that just functionally do not happen outside of like kitplanes or formation flying. Nobody wears a chute because if you're in a scenario where jumping is even an option, you're almost certainly in a situation where gliding to a relatively safe landing site is a better option.

Glider pilots wear parachutes because the nature of the sport introduces a significant mid air collision risk in contest conditions with many gliders sharing chunks of sky. And outside those conditions, you have to have one on to sit in the seat, so you wear it anyway. In power planes, only if you are doing aerobatics, because the risk of the thing just falling apart on you just flying around in 1-2G flight is too small. Do you wear a crash helmet when you go up and down stairs?


text:
How do you find the center of gravity of an Anthropomorphic Test Device
In regards to the aircraft seat testing process in which the occupant's center of gravity is required, I am quite interested to know how people calculate the C.G. of an Anthropomorphic Test Device (commonly known as crash test dummy) It would also be really helpful if I could get relevant resources in the answer.

Finding the CG (barycenter) of anything (rigid) can be done by hanging the object at successive points. The CG is at the intersection of the vertical lines crossing the points. 2 points in 2D, 3 points in 3D.

@mins I was about to Post exactly that but are you saying we can be sure the CG won't change if one arm is extended, or the legs tucked up?

@RobbieGoodwin: In a constant gravity field, the CG is the barycenter of mass, if any change occurs in the shape or in the density, the CG location changes, and a new determination is required of course. That's why the CG can't be provided as a single location by the manufacturer. This method is similar to the one used to balance wheels, except the wheel is rotated to use momentum/torque in place of weight to detect smaller CG offsets.

I would expect that the manufacturer would be able to provide you with the location of the CG among the specifications of the dummy.
Unfortunately, the CG of a flexible object depends on the shape the object is taking at the moment.  The CG is obvious for a uniform bar of steel, but bend it into the horseshoe and the CG is no longer located within the bar.
If I had an oddly shaped rigid object that I wanted to find the CG of, I would hang the object from multiple points.  Draw a vertical line down the object from each hanging point.  Where the lines cross is the CG.

The center of gravity of a standing human body is roughly about an inch below navel:
https://www.sciencedirect.com/topics/physics-and-astronomy/center-of-gravity
For a seated person the CoG is shifted forwards and up of navel, total distance and angle will vary according to the specifics of the posture:
https://www.researchgate.net/figure/Center-of-gravity-of-a-person-in-sitting-posture-Body-segmental-Lengths-are-reported-in_fig2_256203106
The antrophomorphic test dummies mimic the proportions, weight distribution and kinematics of a human body as close as possible, for obvious reasons: to accurately replicate the response of a human body to forces, the ATD must resemble it accurately, not just as a whole, but down to its individual part also.
Example information of THOR ADT: Anthropomorphic Test Devices; THOR 50th Percentile Adult Male Test Dummy; Incorporation by Reference
If you wish to build a more accurate model, here is an example of a further break-down of proportional masses of average human body parts (Source):

Segment
Male
Female

Head
8.26%
8.20%

Whole Trunk
55.1%
53.2%

Thorax
20.1%
17.0%

Abdomen
13.1%
12.2%

Pelvis
13.7%
16.0%

Total Arm
5.70%
4.97%

Upper Arm
3.25%
2.90%

Forearm
1.87%
1.57%

Hand
0.65%
0.50%

Forearm & Hand
2.52%
2.07%

Total Leg
16.7%
18.4%

Thigh
10.5%
11.8%

Leg
4.75%
5.35%

Foot
1.43%
1.33%

Leg & Foot
6.18%
6.68%

For some aviation-related purposes (but not necessarily the OP's), it might also be useful to know the CG location of a sitting human body-- but not surprising that your sources didn't cover that-- plus obviously would be dependent on how upright the seat back was, etc--

Thanks for the heads up, table fixed. As for the seated posture: exactly as you wrote, there are a lo of variables; angle of seat, position of hands and legs etc. I added one "version" of this case.

Awewsome answer!  TY

The center of gravity of a standing human body is roughly about an inch below navel:  is of course also true of a prone or supine human.  That's testable with gym equipment such as a foam roller if your abdominal muscles are in decent condition.

Yep, and also for left and right alteral recumbent and any other straight posture regardless of orientation : ) That was just how it was described on the site. As a sidenote I was contemplating using "erect" instead of "standing", but...

There is no more need to calculate the CG of an Anthropomorphic Test Device than there is to calculate the actual CG of a live body.
Aircraft weight and balance calculations generally don't require that level of precision. Roughly centered on the seat, "X" inches from the datum is close enough.

More precision is needed for weight-shift trikes, hang gliders, and the like.  (But I've no idea how that would connect with the question's tag accident-investigation.)

@CamilleGoudeseune, it isn't just the tag, it's the central theme of the question.  Are you saying that crash test dummies are used to test hang gliders?

Tee hee!  No, I'm just lacking context for the question.

Yes, I agree that understanding "why" helps to answer appropriately...

the main question is regarding the aircraft seat testing process in which you would require the location of CG of the ATD, I couldn't find a tag specific to that

@SG-005, Interesting...  I am curious why, because the human body comes in an infinite variety of physical forms compared to a factory dummy, and it would seem that an average sized ATD made of material with a uniform density approximately equal to meat and bone would be sufficient.  What do you do with the CG data?  P.S.  You haven't accepted an answer or commented on others yet, are you getting any useful information?


text:
Why do similar Airbus and Boeing aircraft use different flap and slat mechanisms?
Why do most Boeing aircraft and their corresponding Airbus counterparts have different flaps and slats mechanisms (747 and A380, 737 and A320)? I'm assuming a lot of these mechanisms are patented. For example, the 747 is equipped with Krueger flaps while the A380 has the leading edge droop as slats.

Example of Kreuger flaps on a 747

If I had an answer for you, I'd be happy to do so. I had to go look up Kreuger flaps and droop and slats, so I added links in while I was at it. Honestly, I'm still debating whether that's a droop or Krueger flap in your picture - my reading least me to believe it's a flap, but the shape of the leading edge says droop (to me, at least).

Its actually the Krueger flap of a 747.

I'm not sure if there's a good answer to this because it's like asking why BMW and Audi use different components. It's just what those companies decided to use as part of their overall design: there's more than one way to implement most things.

@Pondlife, I completely agree that there's more than one way to implement things. But there is always a reason for doing/implementing something. For example, there must be a reason for the 747 having Krueger flaps, and the A380 having drooping slats. I'm not sure what the actual reason may be, but possibly because one creates more lift on an airfoil than the other.

@user17772: There's actually a difference between the two - slats move forward and down, opening up a slot between the slat and the rest of the wing (this is why permanently-deployed slats, found on some STOL aircraft, are also called "slots"), whereas droops just swing down, without moving forward or opening up a slot.

Isn't a large part of the answer simply "patents"?

While the aircraft you mention would appear to be similar, this is not really entirely true. Specification wise yes, but the period it was designed in has a huge impact. For instance:

The first flight of the Boeing 737 was in 1967, with the Airbus A320 following considerably later in 1987. (20 years later)
Boeing 747 first flew in 1969, the Airbus A380 followed in 2005. (36 years later)

Each redesign is expensive and time-consuming so if there is no conceivable benefit it is not done. There is no clear cut equation on when a certain type of slat and flap mechanism is chosen:

The operating environment changes over time. One important consideration is the landing speed (and corresponding runway length). This for instance limited the Boeing 707 greatly, and it was likely that Boeing might not wish to run into the same problem with the B747 to enable it to serve as many markets as possible at a time when airports were not so advanced. For this reason they put in the very complicated triple-slotted flaps. (For comparison, look at the A380 which has caused headaches as airport operators have to modify and/or rebuild facilities to accommodate it)
Aircraft have become lighter over the years with the introduction of new materials, production techniques and better optimized engineering.
Technology has become better at modelling, so we can be less conservative in design choices as there is a better idea of the end result. This applies not only to the structure but also aerodynamics.

The bottom line is that you take the values and look at a few different designs. You choose the one that best fulfills your criteria (not only in performance but also what you can build) and go with it, and this choice would change over time. If the Boeing 737 would be built today, the details most probably be fairly different.
What does this mean?

The A380 for instance only has single slotted slats. When the B747-8i recently received a new wing design, it shifted from triple to double and single slotted flaps.
Both the A350 and B787, designed roughly around the same period, both have the rear single slotted flaps and (to large extent) the same slat design.

Most recent aircraft use a flap/slat design which is as simple as possible to reduce the maintenance cost. The tripple slotted flaps seen on the 747 look awesome but are a pain to service.

Generally, a series of related aircraft uses the same wing: Take the A318, A319, A320 and A321: All use the same wing with the same airfoil, but the much lighter A318 can get away with single-slotted flaps while the heaviest, the A321, needs two flap sections for a double-slotted flap to tease out every bit of lift in order to keep the increase in take-off and approach speeds small. Since the flaps are the easiest part of the wing to adjust its lift capabilities to the needs of the aircraft, you will see a lot of variations in them.
The leading edge devices don't show so much variation: Inboard, a Krüger flap is most common, and outboard a slat is used. While a slat will always have a slot between itself and the wing, a Krüger flap can be slotted or unslotted. When extended, both will show a very similar shape to the airflow, and only their movement is different: While the slat moves forward with a small downward rotation, the Krüger flap swings around a hinge close to the wing's leading edge. Both have either a fully retracted or a fully extended position, while trailing edge flaps can be set in steps to adjust them for moderate lift / low drag increase during take-off, or high lift / high drag for landing.
The complex hinge mechanism in your picture allows the Krüger flaps of the Boeing 747 to increase their camber when extended. This is actually unique among airliners.
The Krüger flap is the newest of them and was invented in 1943. All patents on them expired long ago, and only individual details like the mechanism might be covered by patents today.

IIRC A321 actually has slightly (4 m²) larger wing than the three smaller variants.

@JanHudec: Interesting - the Wikipedia link gives the same 122.6 m² for all four, but lists a smaller tail for the A318. Airbus itself does not even list such mundane numbers as wing area.

I remember reading it somewhere, but I can't find it now either. I have downloaded some PDFs from Airbus (Airplane Characteristics for Airport Planning; separate document for each type) that have more dimensions, but don't have the wing area either. But they do have different wing span for A318/A319/A320 and A320, 34.10m and 34.15m respectively for the older variant with the small wing fences (for the variants with winglets it shows the same span 35.48m). A321 is the only where the winglet variant got new series number, so maybe they switched to the common wing for it?

A321-100 also has different fuel tank layout without the split to inboard and outboard and, somewhat surprisingly, slightly smaller capacity. But I only have document for A321-100, not A321-200, so I can't tell whether they switched to the common wing for NEO or not.

@Peter, from memory, A318 has larger tail (apparently due to its shorter arm).

Why do most Boeing aircraft and their corresponding Airbus counterparts have different flaps and slats mechanisms. For example, the 747 is equipped with Krueger flaps while the A380 has the leading edge droop as slats.

Engines thrust - definitely at least for the B747.

One of the design requirements of the B747 was to have a takeoff length similar to the other jetliners of the era in order to be able to use without problems most of the existing airports. Intuitively, takeoff distance is proportional to thrust and $C_{L_{max}}$, with a longer landing strip required if thrust and/or $C_{L_{max}}$ are small.
When the B747-100 was first developed back in the '60 the only available engine supplying enough thrust was the Pratt&Whitney JT9D. The JT9D was the very first civil turbofan in use and it was actually developed (not without problems) in parallel with the B747. Albeit delivering much more thrust than comparable turbojet of the same era and being also much more efficient, its thrust was just enough to make the B747 fly. Having a limited thrust, the only way for the B747 to fulfill the requirement on the takeoff length was to use high-lift devices as fancy as possible in order to increase $C_{L_{max}}$ as much as possible. This basically locked the choice to triple-slotted flaps on the trailing edge and Kruger flaps on the leading edge (plot source):

Just for comparison we can consider the A340 (better than an A380) which had more or less the same MTOW but much powerful engines: its four RR Trent 556 could deliver 275 kN (62'000 lbf) thrust i.e. some 30% more than  the 206 kN (46,300 lbf) of the original B747-100: a simpler single slotted flap plus slat was enough to get the A340 in the air.

It's funny you use the A340 as a counter-example since the original A340-200/-300 variants using CFM56 engines were famously under-powered as well. Only the larger -500/-600 variants designed a decade later had the more powerful engines. They also got a bigger wing, but I'm not sure if they changed the flap mechanism then?

authors: الرجل الأسطوري


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Are there any FAA written test centers outside of the USA?
I'm Chinese and seeking to be a private pilot, but only have limited time.
I want to finish the FAA written test before I start flight training. Is there a list of international testing centers?

I've checked with PSI support. The answer is no. Only military bases and the USA have FAA written test centers.

Yes. You can find a list of international PSI testing center locations here.

Do these locations offer FAA tests? Not every PSI location does.

It seems outdated. I checked China, South Korea in https://faa.psiexams.com/faa/login and not found any of it on your list

The FAA does not identify if a PSI location administers FAA tests. I could only find a listing of the centers that are located outside the US. It is up to you to find out if you can take FAA written tests there. I am not entirely sure, but I think the FAA has cancelled all written tests outside the US except for US military personnel at some international bases.

You can search for testing locations here. I tried searching a 300 mile radius around a few major cities and got zero results, so it seems that they are rare, if any exist at all. Still, you can search to see if there are any in your area.
The test itself only takes a couple hours, and doesn't need to be done until right before the checkride. So unless your time is extremely limited it shouldn't be too hard to squeeze it in.

Unfortunately, my time is extremely limited. I know this websites but as you can see, they list all countries in the world but most of them is empty. i just want to know if there is an intact list


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What equations and constants did they use to build the E6B flight computer?
E6B Flight computer:

During my practice test, I tried to use the wikipedia provided equations to calculate Density Altitude and ended up with different results than the E6B.  In this case, it looks like the E6B uses something like 130FT/DegC as opposed to posted 118.8 indicated on Wikipedia.
Is there a source for the actual equations and constant values they used for the values on the E6B?

No one knows, because it was magic!

Book on this: Formulas for the E6-B Air Navigation Computer

The Aviation Formulary curated by Ed Williams is probably the best all-around source for the equations.  His going-in assumption is that you have a working understanding of the specific concept and are just looking for the formulas, it is not a tutorial about how to understand the concepts.
Pay special attention to both to his "worked examples" and the excel template that is linked from the introduction section just below the contents.  You have to look for it to find it, but it's a goldmine.
It's a treasure trove and I keep a downloaded copy of each version in case it just goes away someday.

Given the era it was developed and it's Navy origin, I suspect the basis for the E6B atmosphere calculations was NACA Report 218, written by Walter S Diehl of the
Bureau of Aeronautics, Navy Department.
The US Standard Atmosphere was first published in 1958. Later editions of that model and the ICAO Standard Atmosphere differ only above normal altitudes for aviation. The equations you found on Wikipedia are probably based on the US/ICAO standard atmospheres.


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How is the flow of air through a NACA submerged inlet determined?
This question pertains to the design and/or selection of NACA submerged inlets. I am trying to understand what kind of airflow would enter a NACA submerged inlet/scoop and am looking for a good reference to use.
Suppose you have a typical NACA submerged inlet , which has a 2.75 inch diameter hose connection (so an area of 5.94 inch2), and you are flying at 100MPH. How much air actually enters the duct?
I've seen the famous NACA 5i20 Report but can't seem to find the answer or don't understand it fully yet. The report routinely compares the inlet air velocity (Vi) to the free stream velocity (Vo), calling it a ratio "Vi/Vo". It then shows pressure losses for various Vi/Vo ratios. But how do you know what Vi your scoop will get?
I've thought about relating the mass flow entering the duct area (the triangular part) to the mass flow through the inlet, but I'm not sure if that is the most accurate method.

A pity that the authors of that report were unfamiliar with tables of contents.

The flow through the inlet is determined by the resistance of everything behind the inlet. If the inlet is blocked off internally, there is no flow and $v_i$ = zero.
The mass flow is indeed the way to determine through-flow: mass in = mass out. Which is a function of the pressure build-up (both in and out), cross section area (both in and out), temperatures, etc. A similar process occurs in a jet engine inlet.


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When did fighter jet designers stop prioritizing Whitcomb's Area Rule?
According to this source, designers of military fighter jets seemed to have stopped centering their design on Whitcomb's Area Rule for quite a while. Yet even the twin series fighters struggle to break the sound barrier without afterburner in a combat loadout. Then why have the designers stopped caring about transonic drag reduction? Or is it because they never intend to let their jets come near Mach 1, just a little bit below it or use the afterburner and shoot past it? Does this mean newer supercruising fighters, if they intend to fly all the way up to their Mach 1.5-ish cruise speed on dry power, should still care about shaping according to the Area Rule?

The article itself says, "With the advent of more powerful jet engines, “drag rise become less of a factor because you have enough power to bulldoze through it,” says Martin. That’s why designers of the Typhoon or the Lockheed Martin F-35 Lightning II don’t give a moment’s thought to drag rise; engines on those aircraft have plenty of brute force to overcome it."

Of course designers still care about the area rule, but it has become less important. The original area rule is just for Mach 1; at higher speeds you need to look at the cross section distribution along the Mach cones - but which ones? They change with angle of attack and Mach number, and there is no single point anymore where performance needs to be optimized. Rather, the aircraft has to perform well from subsonic to mild supersonic speeds, and the higher speeds for which the older designs from the Fifties and Sixties were optimized turned out to be irrelevant in practical use.
Next, which configuration needs to be optimized? The F-106 carried exactly one type of AA missile. Today, the typical aircraft will carry everything from dumb bombs to laser illumination pods, with fuel tanks and electronic warfare pods thrown in between. With that kind of variety in accessories who can tell which configuration exactly it is which needs to obey the area rule?

To all nitpickers: Yes, I know that the F-106 could also use other missiles, but nuclear warheads on AA missiles were and still are an oddity.

@ymb1 … they are a sign of desperation. This happens when a good design is abused for tasks it was never designed for. And still they are the least drag-inducing way to carry more fuel.

@PeterKämpf then what about the statement in the link about modern engine powerful enough to not care about transonic drag? I got the impression that although the jet engines have become multiple times more powerful than the 50s~60s, very few modern fighters can break the sound barrier on their dry thrust alone. Then what is the precise meaning or implication of this "powerful engine" statement? I get that F106 is the poster kid of the area rule because it needs to concentrate on M=1 drag reduction to be able to break the sound barrier at all, then what about modern fighters?

@MeatballPrincess: Yes, the statement is true. But still - produce a lot of drag and you get punished in consumption, which cuts endurance. So it is not completely irrelevant and designers still try to lower the total drag over a mission as much as possible. If you plan to loiter a lot, your wing will have more aspect ratio. If you plan to fly supersonic, your area distribution still does carry some weight. But it is no longer the overarching principle to which all other concerns must yield.

@PeterKämpf then my understanding is that "powerful" means that fighter can fly somewhere below $M=1$ with lots of excess thrust, even though for example the F15C got almost $0$ excess thrust in $\sim15{,}000,\text{ft}$ at local $M=1$? Is it possible for a supercruising fighter to have to use AB to get over $M=1$ (where it cannot on MIL thrust alone) to its designed supercruising point (say $M=1.5$ where it has $>0$ excess MIL thrust say at $2\mathrm{g}$)?

@MeatballPrincess: Using AB to penetrate the sound barrier saves fuel. However, with increasing speed comes increasing ram pressure in the intake, so the same engine will produce more thrust at a higher Mach number. That is what makes super cruise possible. The excess thrust helps most in subsonic manoeuvring: It allows the highest turn rate without loss of altitude.


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Where can I find detailed dimensions of aircraft panels?
I'm thinking of building some of physical aspects of a flight simulator, such as the overhead panels and pedestal.
Is there a publication available where I could find detailed dimensions of cockpit panel sizes of say Boeing 737 and A320s? I've found some pictures online but they don't quite have the detail I would like. Google images shows a few results with detailed dimensions, so I'm wondering where they got theirs from... actual measurements perhaps? (there are photos of measurements, but i'd like something maybe a little more exact)  Is there maybe a standard size of these panels, also on the larger jets?

Sort of thing I'm looking for:

There appears to be some (free) quality materials accessible through the download section of Open Cockpit's Website, including CAD (.dwg) files and drawings such as this 737 Overhead Panel.
(seem to have at least partially answered my own question, but a clear technical drawing if somebody could provide/suggest a source of one would be greatly appreciated)

You could try the manufacturer's parts or service manuals. The quality and precision of drawings contained in these may not be up to what you're looking for though so best to see if you can look through one before forking over the cash for it...

A good start is to read the free and available military standards regarding Aircrew station design.
MIL-C-6781 : CONTROL PANEL, AIRCRAFT EQUIPMENT, RACK OR CONSOLE MOUNTED
MS25212 : CONTROL PANEL, CONSOLE TYPE, AIRCRAFT EQUIPMENT, BASIC DIMENSIONS
MS25213 : CONTROL PANEL, CONSOLE TYPE, AIRCRAFT EQUIPMENT, TYPICAL INSTALLATION
MIL-DTL-7788 : PANELS, INFORMATION, INTEGRALLY ILLUMINATED
MS25212/MS25213 have drawings like the ones you request. They state that the nominal width of a type I standard panel shall be 5 3/4" 146.05 mm, and that the length of a type I panel shall be a multiple (between 3 and 24) of 3/8" 9.525 mm. They also state that the distance of the fastener holes center from the (nominal) width edges of the panel shall be .1925" ~4.9 mm, (c/c 5,365" ~136.3 mm), and from the height edges .562" ~14.3 mm (c/c multiple of 3/8" 9.525 mm).
Type II control panels are designed for specific purposes where one or more dimensions or features to qualify for being type I control panel cannot be used. Type II control panels shall be similar to type I control panels in all practicable aspects.
Regarding Boeing and Airbus the question is a bit harder. Both appears to derive their design from the military standard but to varying degree. So seems for example Boeing to be using a double standard width overhead center panel of 11 1/2", while Airbus are using a slightly larger dimension of (measured) 325 mm. Thou both are using the same (Dzus/DFCI) quarter turn fastener systems giving the length of the panels.


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How does a propfan work?
Could someone give me a simple working of a propfan? Also I've heard the 2 sets of blades in a propfan rotate in opposite directions. Is this true? And if so why?

You may explicit why ressources such as Wikipedia can't help you.

Go to Wikipedia.  Search ‘turboprop’ and ‘turbofan’

The Wiki explanation were not simple, so that's why asked here :)

To keep it simple (source for all that follows is Wikipedia - Propfan):
You can think of the contemporary propfan as a turboprop that has two high rpm contra-rotating propellers or fans with six or more highly swept fan blades each. These fans are driven by a turbine engine through a reduction gear or a low-speed free turbine.
High rpm is used to reach higher speeds and altitudes than conventional propellers.
The contra-rotating fan arrangement is used to remove torque effect, and to enhance efficiency (note: some early versions had just one fan).
Fan blades are swept to keep drag minimal at high rpm (high blade airspeed).
This description might very well be considered an oversimplification, but I guess this is what was asked for.

Thx! The answer you gave is perfect!


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Two Pilots Logging Dual PIC
I have a question regarding two pilots, Pilot A who flies under the hood, and Pilot B who is Safety Pilot. According to the FAA Hicks Letter, Pilot B can be made in charge of safety of flight to log the entire flight as PIC time; but my questions are:

if Pilot B is 'acting PIC' and Pilot A is the 'sole manipulator of the controls' then is the 0.2 of every flight able to be logged by Pilot A?
Once the hood is taken off but Pilot A is still the sole manipulator of the controls, does this count to log as PIC time?
Also if the Safety Pilot is acting PIC, who logs the X/C time? (0.2 being Takeoff and Landing out of a 1.0 total flight time).

Edit
I see what you all are saying, I just have one more follow up question regarding the FAA Hicks letter. It states:

Therefore, while it is not possible for two pilots to act as PIC simultaneously, it is possible for two pilots to log PIC flight time simultaneously. PIC flight time may be logged by both the PIC responsible for the operation and safety of the aircraft during flight time in accordance with FAR 1.1, and the by the pilot who acts as the sole manipulator of the controls of the aircraft for which the pilot is rated under FAR 61.51.

It then also states later that:

However, the two pilots may, prior to initiating the flight, agree that the safety pilot will be the PIC responsible for the operation and safety of the aircraft during the flight. If this is done, then the safety pilot may log all the flight time as PIC time in accordance with FAR 1.1 and the pilot under the hood may log, concurrently, all of the flight time during which he is the sole manipulator of the controls as PIC time in accordance with FAR 61.51(c)(2)(i).

So with the Hicks letter putting it this way, shouldn't Pilot B also be able to log the 0.2 of the flight as acting PIC as the Hicks letter states, while Pilot A is sole manipulator of the aircraft and logs it as well?

The acting PIC can only log time while there are two required flight crewmembers. So, assuming the flight does not otherwise require two pilots, they can only log time while the sole manipulator is under the hood.
Only the sole manipulator can log cross country time. To log cross country time, the pilot in question must be a required crewmember for the entire flight. The acting PIC is not a required crewmember for the takeoff and landing and so cannot log cross country time.


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What throttle setting does an airliner use while cruising?
As a passenger, I have always felt like the engine are not kept at take-off thrust/full throttle a few (tens of) seconds after take-off, and so for obvious reasons as fuel efficiency, engine wear prevention, noise control for population in airport area, and so on.
I have also read on this site or on excellent Twitter account of the A380 captain Dave Wallsworth lots of explanation about how to reduce the take-off thrust (eg. flex temp).
But I cannot find any information about the throttle level (N1 percentage may be more accurate) during cruise. My guess is it is not constant because of weight decreasing during flight due to fuel comsumption, but this is only a guess.
I know Airbus airliners have a "CLIMB" position on throttle which allow the autopilot to set N1 automatically to reach cruise speed, but I would like to know how N1 value evolves during cruise.
A320 throttle quadrant with lever at CLIMB position, with auto-throttle range:

(source: skalarki-electronics.eu)

From my experience in B767, the computer generated flight plan (with preset company settings) will give a Cruising Mach number, eg we used .80. At the initial cruise level the auto-throttle will set the power required to achieve that speed.
As the a/c burns off fuel, the required thrust and  therefore N2, N1 and fuel flow will decrease to keep the same Mach number.


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Why am I seeing extremely high L/D ratio of (e.g. L/D = 90) in my 2D airfoil analysis?
Can anyone explain/justify why, using the 'Theory of Wing Sections' by ABBOTT the lift/drag ratios seem so high. For example, I'm doing an analysis of the NACA0012 airfoil and using their experimental data on pg462/463 at 8 degrees CL is roughly 0.9 and CD is roughly 0.001 giving a lift/drag ratio of 90. This seems incredibly high since as far as I'm aware lift/drag ratios go up to 70 to 75 practically from answers I've seen on here. My problem is that my CFD analysis also gives these high lift/drag ratios but I just didn't think it was possible.
Thanks in advance to anyone who can help!

I would replace "possible" with "practical"

Could you be looking at lift/parasite drag only ratios? Induced drag depends on span only, so probably isn't included in 2-D section analysis.

@JanHudec yeah I suppose it could be that not all the drag is included since it's a 2D airfoil, are airfoil L/D g ratios generally larger then?

@Barnaby, the lift is approximated well by the 2-D analysis, so if any form of drag is not calculated, the L/D will obviously be higher than the final 3-D wing.

The difference is that one figure looks at 2D flow around an isolated airfoil, where L/D ratios of up to 200 can be reached, while the other figure is for the 3D flow around a whole airplane, warts and all. The additional sources of drag which are absent in 2D flow around airfoils are:

Induced drag. This will double the total drag right at the polar point of best L/D.
Fuselage and tail drag. Of course isolated airfoils only have their own surface where friction drag occurs, but all the non-lifting parts on whole airplanes will bring D up without helping L.
Lift loss at the tips. L/D for airfoils uses the pressure distribution and maybe a drag rake at the windtunnel center section, so no tip effects will tarnish the result, but the outer wings of airplanes will have reduced lift relative to the local surface and drag because of tip effects and to improve stall behavior.
Trim drag: The tail can very well add a downforce to balance the lengthwise lift distribution with the location of the center of gravity. Now extra lift has to be produced by the wing, and both, downforce at the tail and extra lift on the wing, will produce more drag.


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Could TCAS be used as a backup source for position, attitude or air data?
Given the number of incidents related to freezing pitot tubes, is there any current/planned implementation to infer aircraft position/speed/attitude/bearing based on TCAS data as a last resort data point? If not, is this at least doable?
Even if TCAS gives only relative data, one may correlate the data at the last moment before pitot data are lost to assist in dead reckoning.
Edit: I am not sure if my question has been misunderstood, or that I am misinterpreting the answers. The question is more like inferring current aircraft's data from transponders of surrounding aircrafts. Let's suppose at time $t_1$ I have planes A, B, and C within my TCAS range. I will possess information regarding their bearing, distance, speed, and altitude. At time $t_2$, even without my own altimeter, I can at least infer my own vertical speed using trigonometry. A, B, C are like ocean currents, a background moving together with me. If I know enough information about them, depending on what TCAS's underlying data stream provides, I can probably infer my own position, altitude, speed, and heading. From my limited understanding, TCAS bearing, distance, and speed are relative to me. However if I have not mistaken, despite what is shown on the screen, altitude is absolute measurement from the other planes' altimeter. One side fixed on a similar triangle, you'll get the rest.

Indeed it is amazing, the possibilities in FMS software development.  I would think ADS-B could provide more data to the FMS than the limited scope of TCAS.  Unless there's at least three other aircraft within scope, there'd be no data.  Oceanic service would be nill for either.  Here's FlightAware's coverage of ground-based ADS-B receivers:  http://flightaware.com/adsb/coverage

GPS might be an option, combined with jetstream and wind plotting.

And EGPWS contains a map of all of earth and knows the aircraft position and terrain altitude.

GPS inferred data won't give you airspeed

@STWilson  You've got it backwards.  The FMS provides much the data that is broadcast via ADS-B Out.  While it is possible to send ADS-B In data to the FMS there are no FMS functions that can make use of the data.  There are some proposed "Surveillance Applications" that use the ADS-B In data that might have an interface to the FMS, but what it can and is allowed to do has to be defined and agreed upon by a large number of organizations.  The FMS just guides the plane along the flight plan and the flight plan has to stay in sync with the ATC clearance.

@Gerry  I refer to the increasing potential of flight management systems, specifically the FMS software, to processes data from the variety of IO devices like GPS, air data/system computers, AHARS, transponders, ADS-B, XM-feeds, et al.  I refer to FMS that today does more than output to the flight director.

@STWilson Since my day job involves developing said FMS, I can authoritatively say that while the FMS does process data from a large number of sensors, it does not receive any input from ADS-B nor is any planned in the near term.  The FMS does have an output to the ADS-B Out function (usually the Mode S transponder) that provides "intent" data (aircraft state and flight plan), but that's it.

If there is something to use when pitots are blocked, this is the attitude indicator. Just keep the recommended attitude corresponding to the throttle position, as written in the FCOM.

Question edited with clarification. Hope it makes more sense. Chiming in the discussion above, it is quite sad that despite so many data sources are available, FMS fails to integrate them.

@Mobile Computing, understand that the TCAS sensor gives:1. Distance (rather accurately, using transponder return time), 2. Bearing (rather crudely and 3. Pressure altitude of the observed aircraft.  Do we agree that this is the fundamental capability of the TCAS sensor?

@mongo Yes, and?

If we agree that is the capability of the sensor then there is no additional information available which would assist in DR or navigation.  Integration of OTHER sensor data (GPS, ADS-B etc might yield usable data, but the bottom line is that TCAS alone provides no useful data for dead reckoning. While there might be an instant where you suggest that an inference could be made, most failures are not detected instantly, and by the time they are detected other aircraft have moved.

Can TCAS assist in dead reckoning?
First, understand that TCAS runs off secondary (transponder) radar data. Essentially your aircraft with TCAS is like ATC radar, interrogating other transponders, getting their pressure altitude and getting their bearing from your aircraft.  That data combined is processed to create threat advisories.
If displayed on a moving map, the moving map is typically fed positional data for your aircraft from GPS, and the TCAS alert data is overlayed onto the map.
So to address your question, TCAS only provides data about transponders relative to your aircraft.  TCAS does not determine where you are, or where you are going. (Depending upon your display it might appear that way, but that is not part of the TCAS system.)
So the answer to your question is no, TCAS does not effectively create a secondary navigation capability.
Addendum #1
TCAS can only sense altitude through the host aircraft's static port, so if it is iced over, TCAS has invalid altitude information.
TCAS does not sense other air data, such as pitot pressure information (airspeed).
TCAS does not know the host aircraft's position.  Any positional information in TCAS is derived from pressure altitude information the host and all interrogated transponders.  If the host static port is obscured, the information on threats will be invalid, because TCAS displays relative to the host pressure altitude.
In other words, there is no absolute positional information, and all TCAS information is relative to the current host aircraft, and derived from the secondary beacons of interrogated aircraft transponders.

Afair:
The ATC/SSR antena can barely discern azimuth on the responses, and effectivelly no elevation data.The only accurate info from TCAS is distance to intruder (and closure rate). This means you have some ideea of the direction of the intruder (not accurate enough mind you ,which is why RAs have only vertical guidance), but no real means of determining their relative altitude to you.
So basically you know you have another aircraft somewhere to your left, at FL110... how could you infer your altitude from that?
GPS altitude is another issue, and ,in fact, backup altitude from GPS does appear on some (A320s at least) PFDs when all ADR data is lost .
Later edit: if you bring ADS-B into the question, with a/c broadcasting their lat/long and flight level, you know the distance from your a/c to these points and can draw a sphere arround each of these points, at their intersection you can triangulate your position (including altittude) assuming you have enough traffic arround (min 3). Mathematically ,yes, posssible, but once you factor in the delays in ASD-B, the rounding errors in transmitting FLs, etc the accuracy would be horrible probablly within 100s and 1000s of feet


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Customs at refueling stops?
This question has me wondering: do passengers/crew have to to through customs at refueling stops?
Does it matter if they stay in/with the aircraft vs. going into the FBO, e.g. to use restrooms?
If it is different for different types of flights (e.g. airline vs private), please answer for each.

One data point: flying with Qantas from Sydney to London and back, the aircraft stops at Dubai. Passengers had to disembark and then pass through security to return to the gate to re-board the aircraft. We didn't pass through immigration or customs. The situation would certainly be different in the US, and perhaps at other airports around the world.

It depends on the country and may depend on the airport - you have to check the AIP. The term "technical stop" or "tech stop" is often used instead of "refuelling stop", since I guess there could be other reasons than refuelling why you'd need to land and takeoff without exchanging passengers or cargo.
As an example from personal experience flying my 2-seat experimental across the pond, Kangerlussuaq and Narsarsuaq in Greenland do not require crew to go through any customs or immigration checks for a technical stop, while Reykjavik in Iceland does. They let me get out of my drysuit and take a pee break first though, unlike the US CBP!